During this reporting period the Cameca Electron probe has been extensively modified to enhance its ability to perform biological microanalysis. Solid state backscattered electron detectors have been installed on the objective lense pole piece. These detectors permit the observation of biologically interesting elements, greater in atomic number than ll from specimens. An energy dispersive x-ray detector and associated computer controlled multichannel analyzer have also been installed. This x-ray detector permits the observation and quantification of elements over most of the periodic table (Z greater than 9). Finally, the existing wavelength dispersive x-ray detectors have been automated and data acquisition can be accomplished using our laboratory PDP ll-60 minicomputer. A substantial collaborative effort is underway with the clinical pathology branch in the Clinical Center. The work involves the quantification of the magnesium content of individual lymphocytes. Various specimen preparation techniques have been evaluated as well as the mathematical procedures which will be used for the quantification. This work is part of a larger study to provide a quick screening method for the condition of the human cardiovascular system.